JP-A-2000-58226 describes a method for manufacturing an insulator for a spark plug. This method is for manufacturing an insulator for a spark plug, which insulator has a through hole for inserting a center electrode and a terminal electrode extending in an axial direction. The insulator further includes a thick-walled portion having large wall thickness defined between the through hole and an outer peripheral surface.
In this manufacturing method, first, as a preparing step, a press pin used to form a through hole, and a forming die having a cavity are prepared. On the base end side of the press pin, a rib-shaped pin-side spiral portion is formed on an outer peripheral surface.
Also, as a press pin arranging step, the press pin is arranged within the cavity by advancing a leading end of the press pin in the axial direction. Next, as a powder filling step, a raw powder is filled into the cavity in which the press pin is arranged. Then, as a pressing and forming step, the raw powder within the cavity is pressed along with the press pin, and a green body is obtained. A base end of this green body is formed with a green body-side spiral portion to which a pin-side spiral portion of the press pin is transferred.
After the pressing and forming step, as a die releasing step, a green body along with the press pin is released from the cavity. After the die releasing step, as a press pin removing step, the press pin is withdrawn with respect to the green body while being rotated around an axis, and the press pin is extracted from the green body. Then, as shown in FIG. 11 of JP-A-2000-58226, as an unnecessary portion removing step, an unnecessary portion is removed from the green body. In this case, the green body-side spiral portion remains at the base end of the green body after the unnecessary portion has been removed. This unnecessary portion removing step may be performed by temporarily sintering the green body after the press pin removing step. The green body obtained in this way is finished to an external shape corresponding to an insulator for a spark plug.
Then, the green body is sintered at a temperature of 1400 to 1650° C. Thereby, a pin hole formed by the press pin becomes a through hole. Then, the sintered body is finished and sintered by applying glaze to the surface thereof, whereby an insulator for a spark plug is obtained. In addition, as shown in FIG. 1 of JP-A-2000-58226, the green body-side spiral portion remains at the base end of the through hole. This insulator for a spark plug becomes a spark plug in which a center electrode, a terminal electrode, a metal shell, a resistor, etc. are provided. The thick-walled portion of the insulator for a spark plug is located within the metal shell. This spark plug is attached to an engine at a thread portion of the metal shell, and is used as an igniting source for an air-fuel mixture to be supplied to a combustion chamber of the engine.